Method and device for retransmission of transmitted units

ABSTRACT

The invention relates to a method for retransmitting a transmission unit ( 12 ) via an air interface in a wireless access system employing fragmentation for transmissions, wherein the available transmission capacity on said air interface is variable, and wherein said transmission unit ( 12 ) was transmitted a first time together with fragmentation information. In order to enable such a retransmission, it is proposed that the transmission unit ( 12 ) is retransmitted after a refragmentation with information FC, FSN on this refragmentation and with at least some of the fragmentation information FC′, FSN′, TSN that was transmitted before with said transmission unit ( 12 ) in the first transmission. The invention equally relates to a corresponding wireless access system and to a corresponding transmitting unit for such a wireless access system.

FIELD OF THE INVENTION

The invention relates to a method for retransmitting a transmission unitvia an air interface in a wireless access system employing fragmentationfor transmissions. The transmission capacity on the air interface isassumed to be variable in this system and a transmission unit that is tobe retransmitted was transmitted a first time together withfragmentation information. The invention equally relates to atransmitting unit for such a wireless access system and to acorresponding wireless access system.

BACKGROUND OF THE INVENTION

A wireless access system employing fragmentation and an adaptivemodulation for transmitting signals is specified e.g. in the IEEE draftP802.16/DS-2001: “Local and Metropolitan Area Networks—Part 16: StandardAir Interface for Fixed Broadband Wireless Access Systems”, which isincorporated by reference herein. The standard specifies the airinterface, including the medium access control layer (MAC) and aphysical layer (PHY), of fixed point-to-multipoint broadband wirelessaccess systems providing multiple services. In this system, MAC SDUs(service data units) that are to be transmitted from a transmitting unitto a receiving unit are fragmented for transmission into MAC PDUs(protocol data units). The defined system operates at frequencies of10-66 GHz.

An amendment 802.16a of this standard is currently under development forsystems operating between 2 GHz and 11 GHz. The amendment introduces newfunctionality for this frequency range, such as an Automatic RepeatRequest (ARQ). With ARQ, a receiving unit may request a retransmissionof transmission units that were for instance lost during transmissionover the air interface by transmitting an ARQ feedback to thetransmitting unit. The proposed algorithm for repeating a transmissionis a selective repeat algorithm, i.e. only those fragments areretransmitted which are not received satisfactorily at the receivingunit after the first transmission. A selective repeat ARQ mechanism willoften require a packet reordering.

Variations of the environmental conditions on the transmission path arecompensated in a system according to the 802.16a standard with avariation of the modulation and with a forward error correction (FEC).The transmission capacity of the radio link can thus vary significantlybetween the time of a first transmission and the time of aretransmission. In case the environmental conditions deteriorate, itmight be necessary for the retransmission to refragment the fragmentsthat are to be retransmitted into shorter MAC PDUs. The conventionalmethod of sequence numbering employed for a single fragmentation is notsuited for handling such a refragmentation.

The currently proposed solution for refragmentation in the 802.16asystem mandates that the MAC SDUs are chopped into fixed sized ARQblocks. Only the last block will be a shortened block if the MAC SDUsize is not an exact multiple of the ARQ block size. Each ARQ block isidentified by an ARQ block number assigned to it by the MAC. A set ofARQ blocks that are to be transmitted or retransmitted are included in aMAC PDU. The block number of the first block is carried in a subheaderin each MAC PDU, or in each packing element in case a MAC PDU carriespacked MAC SDUs or MAC SDU fragments. The MAC level fragmentationfunction is restricted to fragment MAC SDUs on ARQ block boundaries.

The restriction put on the fragmentation protocol in the currentproposal, however, is incompatible with the fragmentation procedurespecified in the 802.16 standard, since it does not allow a variablefragmentation. The block numbering scheme also limits the possibilitiesof using implementations designed for a 802.16 system also for 802.16asystems.

SUMMARY OF THE INVENTION

It is an object of the invention to enable a retransmission of atransmission unit in a wireless access system which has a variableavailable transmission capacity and which employs fragmentation fortransmissions. It is moreover an object of the invention to enable aretransmission of transmission units which is compatible with thefragmentation protocol defined in the IEEE standard 802.16.

The objects of the invention are reached with a method forretransmitting a transmission unit via an air interface in a wirelessaccess system employing fragmentation for transmissions. The availabletransmission capacity on the air interface is variable, and thetransmission unit is transmitted a first time together withfragmentation information. It is proposed that the transmission unit isretransmitted after a refragmentation with information on thisrefragmentation and with at least some of said fragmentation informationthat was transmitted before with said transmission unit in said firsttransmission.

The objects of the invention are equally reached with a transmittingunit for a wireless access system which transmitting unit comprisesmeans for realizing the proposed method. Finally, the objects of theinvention are reached with a wireless access system comprising such atransmitting unit and a receiving unit for receiving transmittedtransmission units and for requesting a retransmission of a transmissionunit if necessary.

The invention proceeds from the idea that in case a transmission unitwhich has to be retransmitted comprises on the one hand fragmentationinformation of the original transmission unit and on the other handfragmentation information describing the fragmentation employed forretransmission, an easy refragmentation mechanism is enabled whichallows to rebuild and reorder the fragments as required for aretransmission. In particular, the fragments of a transmission unitresulting in refragmentation can be assigned their own fragmentationinformation as any transmission unit which is transmitted for the firsttime.

It is to be understood that the proposed method does not require thateach transmission unit that is to be retransmitted is refragmented intonew fragments. For example, if the available transmission capacity isthe same for a first transmission and for a repeated transmissions of atransmission unit, the transmission unit may be transmitted againwithout the necessity for a different fragmentation than before.

It is an advantage of the invention that it provides a simple solutionfor retransmitting transmission units.

It is moreover an advantage of the invention that it provides atransmission unit numbering scheme that allows the reuse of the 802.16fragmentation protocol. The invention further allows implementationsmore easily to address both, the 802.16 standard and the amendment802.16a.

Preferred embodiments of the invention become apparent from thesubclaims.

Since traffic is usually to be transmitted bi-directionally, and sincein most of the remaining cases at least a feedback is to be enabledafter a transmission, the transmitting unit is advantageously part of afirst transceiver device. Accordingly, the transmitting unitadvantageously transmits the transmission units to a receiving unitwhich is part of a second transceiver device.

Preferably, a retransmission of a transmission unit is carried out uponrequest by a receiving unit, e.g. because the receiving unit notes thata transmission unit is missing or that a transmission unit was receivedwith too many errors. The request for a retransmission can be forexample the selective ARQ mechanism proposed in the above mentionedamendment 802.16a.

The transmission unit of the invention can be in particular a MAC SDUtransmitted in a MAC PDU without packing, a MAC SDU fragment transmittedin a MAC PDU without packing, a MAC SDU transmitted in a MAC PDU withpacking, or a MAC SDU fragment transmitted in a MAC PDU with packing.

Preferably, fragmentation information is included in a subheader of eachfragment that is to be transmitted. For wireless access systems that arebased on the IEEE standard 802.16, this subheader has preferably an ARQsubheader format that consists of the fragmentation or packing subheaderdefined in this standard and of an additional part. The additional partmay comprise for example a field extending the fragmentation sequencenumber (FSN) field present in the 802.16 Fragmentation Subheader andPacking Subheader, an FSN′ field replicating the FSN field of therespective first transmission unit, an FC′ field replicating thefragmentation control (FC) field in the respective first transmissionunit, and a transmission unit sequence number TSN field.

The fields employed for a fragmentation control FC, FC′ preferablycomprise 2 bits. The values assigned to the fragmentation control fieldsFC and FC′ may correspond for example to the values defined in the abovementioned IEEE standard 802.16. In this standard, a value of 00 is usedfor transmission units that are not fragmented, a value of 01 for arespective last fragment, a value of 10 for a respective first fragment,and a value of 11 for each middle fragment.

The fields employed for a fragmentation sequence number FSN, FSN′preferably comprise 5 bits. The fields for the transmission sequencenumber TSN preferably comprise 7 bits.

It is to be noted that the lengths of the fields and the codings in thefragmentation control fields can be selected differently. However, theselengths and codings ensure that the method and the system according tothe invention comply with the definitions of the IEEE standard 802.16.

The invention can be employed in particular, though not exclusively, inwireless local area networks (LAN) or metropolitan area networks (MAN).

The invention is moreover applicable for example in point-to-multipointwireless broadband access networks and in point-to-point wirelessbroadband access networks, but equally in other system.

The invention can further be employed in particular, though notexclusively, in systems in which the transmission capacity may changedue to an adaptive modulation which is applied to signals that are to betransmitted.

BRIEF DESCRIPTION OF THE FIGURES

In the following, the invention is explained in more detail withreference to drawings, of which

FIG. 1 illustrates an embodiment of the method of the invention in afirst situation; and

FIG. 2 illustrates an embodiment of the method of the invention in asecond situation.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 and FIG. 2 illustrate transmissions and retransmissions oftransmitting units in a broadband wireless access system according tothe invention. The wireless access system employs fragmentation and anadaptive modulation for transmissions from a base station of a networkof the system to a subscriber station of the system.

FIG. 1 depicts in a first row two MAC SDUs #1 and #2 which are to betransmitted by a base station via the air interface to a subscriberstation. Both MAC SDUs are fragmented for a first transmission into MACPDUs 11-15, which are depicted in a second row of FIG. 1. The first MACSDU #1 is fragmented into two MAC PDUs 11, 12 as indicated by arrowsbetween the first and the second row. The second MAC SDU #2 isfragmented into three MAC PDUs 13-15, which is equally indicated byarrows.

The number of fragments into which the MAC SDUs are fragmented dependson the size of the respective MAC SDU and on the size of the availableMAC PDUs. In case of good conditions, a modulation is selected thatresults in larger MAC PDUs and that thus requires less fragments than amodulation employed for bad conditions.

Each of the MAC PDUs 11-15 comprises a subheader with five fieldscontaining fragmentation information.

The first field in each subheader is a TSN field of 7 bits, which isemployed for identifying the respective MAC PDU. The second field is anFC field of 2 bits indicating for the current transmission the positionof the respective PDU among the PDUs comprising the fragments of asingle MAC SDU. The third field is an FC1 field of 2 bits indicating theposition of the PDU containing the same data as the PDU in a firsttransmission. The fourth field is an FSN field of 5 bits containing afragmentation sequence number for the respective PDU for the currenttransmission. The fifth field, finally, is an FSN′ field of 5 bitscontaining a fragmentation sequence number for the PDU containing thesame data as the PDU in a first transmission. The values for the fieldsfor the current transmission are selected as specified in the abovementioned standard 802.16.

The first MAC PDU 11 for the first MAC SDU #1 is assigned a TSN of 0xF1,and an FC of 10, since this MAC PDU 11 is the first fragment of thefirst MAC SDU #1.

The second MAC PDU 12 for the first MAC SDU #1 is assigned a TSN of0xF2, and an FC of 01, since this MAC PDU 12 is the last fragment of thefirst MAC SDU #1.

The first MAC PDU 13 for the second MAC SDU #2 is assigned a TSN of0xF3, and an FC of 10, since this MAC PDU 13 is the first fragment ofthe second MAC SDU #2.

The second MAC PDU 14 for the second MAC SDU #2 is assigned a TSN of0xF4, and an FC of 11, since this MAC PDU 14 is a middle fragment of thesecond MAC SDU #2.

The third MAC PDU 15 of the second MAC SDU #2 is assigned a TSN of 0xF5,and an FC of 01, since this MAC PDU 15 is the last fragment of thesecond MAC SDU #2.

The respective value in the FC′ fields is identical to the respectivevalue in the FC fields for each of the MAC PDUs 11-15, since thetransmission of the MAC PDUs 11-15 in the second row of FIG. 1constitutes at the same time the first transmission. For the samereason, the respective value in the FSN′ fields is identical for each ofthe MAC PDUs 11-15 to the respective value in the FSN fields.

The assembled MAC PDUs 11-15 are transmitted by the base station via theair interface to the subscriber station. During the transmission, thesecond MAC PDU 12 of the first MAC SDU #1, i.e. fragment 0xF2, is lost.This is reported by the subscriber station to the network in an ARQfeedback message. The lost fragment 12 thus has to be retransmitted. InFIG. 1, MAC PDU 12 of the second row is crossed out in order to indicatethe loss during the first transmission.

When a transmission unit is retransmitted, a copy of the originalfragmentation control information and of the original fragmentationnumber is transmitted along with the transmission unit itself. Thetransmission unit that is to be retransmitted is treated like a new MACSDU called retransmission SDU and is inserted at an appropriate positioninto the queue of MAC SDUs that are to be transmitted. Theretransmission SDU undergoes the standard fragmentation process ifnecessary and obeys the same rules for setting the FC and FSN fieldvalues as an SDU which is transmitted for the first time.

In the example of FIG. 1, the applied modulation is not changed betweenthe first transmission and the retransmission and the transmissioncapacity on the air interface stays the same. Therefore, theretransmission can be carried out without refragmentation. This meansthat the MAC PDU 12 with the TSN value of 0xF2 is transmitted again as asingle fragment 21, which fragment 21 is depicted in a third row ofFIG. 1. The relation of this fragment 21 to the second MAC PDU 12 of thefirst MAC SDU #1 is indicated in FIG. 1 by arrows. The values in some ofthe fields in the subheader of the fragment 21 are changed forretransmission, however, in order to enable the subscriber station tomake use of the received, retransmitted fragment 21.

The TSN field in the subheader of the fragment 21 is provided with thesame value 0xF2 as before, since this field identifies the lost MAC PDU12. Also the fields FC1 and FSN′ contain the same values as before,these field containing the information about the lost transmission. Thethree fields TSN, FC′ and FSN′ enable the subscriber station to relatethe retransmitted fragment 21 to the lost MAC PDU 12.

The value of the FC field, in contrast, is changed to 00, since theretransmitted PDU 12 is not fragmented further, fragment 21 thusconstituting the only fragment. In addition, the value of the FSN fieldis incremented according to the conventional rules to 0x09, the lasttransmitted MAC PDU 15 of the first transmission having been assignedthe value 0x0B.

FIG. 2 illustrates the retransmission for a different situation. Theretransmission is based on the same embodiment of the method accordingto the invention as in FIG. 1.

Again, two MAC SDUs #1 and #2 are to be transmitted by a base stationvia the air interface to a subscriber station. The first fragmentationof the MAC SDUs into MAC PDUs 11-15 corresponds exactly to the firstfragmentation in the example of FIG. 1. This is depicted in FIG. 2 byMAC SDUs and MAC PDUs in a first and a second row which are identical tothe first and the second row of FIG. 1. Again, the second MAC PDU 12 ofthe first MAC SDU #1 is lost during transmission, which is reported tothe network by an ARQ feedback message of the subscriber substation.

In contrast to the first example, the employed modulation is changedafter the first transmission to be more robust. With the new modulation,the content of the second MAC PDU 12 of the first MAC SDU #1 can nolonger be transmitted in a single PDU. Therefore, the second MAC PDU 12has to be rearranged for retransmission.

The MAC PDU 12 with the TSN value 0xF2 is refragmented forretransmission into three new fragments 22-24, which are depicted in thethird row of FIG. 2. Arrows relate the new fragments 22-24 to the lostMAC PDU 12 of the second row.

The respective TSN field of the three fragments 22-24 contains again thesame value 0xF2, which identifies the lost MAC PDU 12 that has to beretransmitted. Also the fields FC′ and FSN′ contain the same values asbefore, since these fields contain further information about the lostMAC PDU 12.

The FC and FSN fields in the retransmitted fragments are set accordingto the normal rules. Therefore, the first fragment 22 is provided with avalue of 10, the second fragment 23 with a value of 11 and the thirdfragment 24 with a value of 01 for the respective FC field. Further, thefirst fragment 22 is provided with a value of 0x09, the second fragment23 with a value of 0x0A and the third fragment 24 with a value of 0x0Bfor the respective FSN field.

The refragmentation mechanism of the invention thus makes it possible torebuild and reorder fragments in retransmission.

It is to be noted that the described embodiment of the inventionconstitutes only an example that may be varied in any suitable way.

1. Method for retransmitting a transmission unit (12) via an airinterface in a wireless access system employing fragmentation fortransmissions, wherein the available transmission capacity on said airinterface is variable, wherein said transmission unit (12) wastransmitted a first time together with fragmentation information(TSN,FC′,FSN′), and wherein said transmission unit (12) is retransmittedafter a refragmentation with information (FC,FSN) on saidrefragmentation and with at least some of said fragmentation information(TSN,FC′,FSN′) that was transmitted before with said transmission unit(12) in said first transmission.
 2. Method according to claim 1, whereinsaid fragmentation information on a fragmentation for a firsttransmission comprises at least one of a fragmentation control (FC′), afragmentation sequence number (FSN′) and a transmission unit sequencenumber (TSN).
 3. Method according to claim 1, wherein saidrefragmentation information comprises at least one of a fragmentationcontrol (FC) and a fragmentation sequence number (FSN).
 4. Methodaccording to claim 1, wherein each fragment (21,22-24) of a refragmentedtransmission unit (12) is provided with a subheader comprising for thefragmentation and refragmentation information: a field for afragmentation sequence number (FSN′) assigned to said transmission unit(12) in the first transmission; a field for a fragmentation sequencenumber (FSN) assigned to said fragment (21,22-24) of said transmissionunit (12) for the retransmission; a field for a fragmentation control(FC′) assigned to said transmission unit (12) in the first transmission;a field for a fragmentation control (FC) assigned to said fragment(21,22-24) of said transmission unit (12) for the retransmission; and afield for a transmission unit sequence number (TSN) assigned to saidtransmission unit (12) in the first transmission.
 5. Method according toclaim 4, wherein said fields for a fragmentation sequence number(FSN,FSN′) are fields of 5 bits, wherein said fields for a fragmentationcontrol (FC,FC′) are fields of 2 bits, and wherein said field for atransmission unit sequence number (TSN) is a field of 7 bits.
 6. Methodaccording to claim 1, wherein each transmission unit (12) transmitted afirst time and each fragment (21,22-24) of a transmission unit (12)refragmented for retransmission is provided with a subheader comprisingthe same dedicated fields (TSN,FC,FC′,FSN,FSN′) for fragmentationinformation.
 7. Method according to claim 1, wherein said transmissionunit (12) is one of: a MAC (medium access control) SDU (service dataunit) transmitted in a MAC PDU (protocol data unit) without packing; aMAC SDU fragment transmitted in a MAC PDU without packing; a MAC SDUtransmitted in a MAC PDU with packing; and a MAC SDU fragmenttransmitted in a MAC PDU with packing.
 8. Method according to claim 1,wherein said transmission unit (12) is retransmitted upon a request by areceiving unit of said wireless access system to which said transmissionunit (12) was to be transmitted.
 9. Transmitting unit for a wirelessaccess system comprising means for transmitting transmission units(11-15) and for retransmitting transmission units (12) according toclaim
 1. 10. Transmitting unit according to claim 9, wherein saidtransmitting unit is part of a first transceiver device, and whereinsaid transmitting unit is transmitting transmission units (11-15) to areceiving unit which is part of a second transceiver device. 11.Transmitting unit according to claim 9, comprising: means forfragmenting each data unit (MAC SDU#1, MAC SDU#2) that is to betransmitted via an air interface into at least one transmission unit(11-15) and for providing each transmission unit (11-15) withcorresponding fragmentation information (TSN,FC′,FSN′); means fortransmitting transmission units (11-15) to a receiving unit via said airinterface; means for receiving a retransmission request by a receivingunit requesting that a transmission unit (12) is to be retransmitted;and means for refragmenting a transmission unit (12) for which aretransmission was requested by a receiving unit, and for providing saidtransmission unit (12) for retransmission with information (FC,FSN) onsaid refragmentation and with at least some of said fragmentationinformation (TSN,FC′,FSN′) provided to said transmission unit (12) for afirst transmission.
 12. Wireless access system comprising a transmittingunit according to claim 9 and a receiving unit comprising means forreceiving transmission units transmitted by said transmitting unit andmeans for requesting a retransmission of transmission units (12) ifrequired.